Symbolic experiment step settings and terminations

CHANGELOG.md

@@ -1,5 +1,10 @@
 # [Unreleased](https://github.com/pybamm-team/PyBaMM/)
 
+## Features
+
+- Step `duration`, `period` and `temperature` accept a `pybamm.Parameter` or `pybamm.InputParameter` ([#5744](https://github.com/pybamm-team/PyBaMM/pull/5744))
+- Added `SymbolicTermination`: terminate a step on any inequality over model variables ([#5744](https://github.com/pybamm-team/PyBaMM/pull/5744))
+
 ## Bug fixes
 
 - The `integration` nox session no longer installs the `pydiffsol` extra on macOS Intel CI runners, where it has no working build. ([#5726](https://github.com/pybamm-team/PyBaMM/pull/5726))
@@ -8,6 +13,8 @@
 
 ## Features
 
+- Experiment steps accept a symbolic `duration` (e.g. a `pybamm.InputParameter`), resolved against the inputs passed to `Simulation.solve`, so one built model serves any duration. Drive cycles now repeat by wrapping the step time, rather than being tiled out to a fixed duration.
+- Experiment steps accept a symbolic inequality as a `termination`, e.g. `pybamm.CoupledVariable("Voltage [V]") > pybamm.InputParameter("Voltage hold [V]")`, so a cut-off can be set on any model variable and its threshold supplied at solve time.
 - Added unstructured mesh support (`UnstructuredSubMesh`, generators, and interface coupling) for arbitrary 2D/3D domains. Hexahedra must have planar faces (warped hexes raise a `GeometryError`), and `UserSuppliedUnstructuredMesh` accepts tetrahedral, triangular, and quadrilateral cells only. ([#5687](https://github.com/pybamm-team/PyBaMM/pull/5687))
 - Generalised `VectorField` to N components and added `Component`/`Norm` operators for multi-dimensional vector fields. ([#5686](https://github.com/pybamm-team/PyBaMM/pull/5686))
 - Removed the left sidebar from the documentation home page for a cleaner landing experience. ([#5699](https://github.com/pybamm-team/PyBaMM/pull/5699))

docs/source/api/experiment/experiment_steps.rst

@@ -33,6 +33,11 @@ Custom steps can be defined using either explicit or implicit control:
 Step terminations
 -----------------
 
+Any inequality over model variables can terminate a step:
+
+.. autoclass:: pybamm.step.SymbolicTermination
+    :members:
+
 Standard step termination events are implemented by the following classes, which are
 called when the termination is specified by a specific string. These classes can be
 either be called directly or via the string format specified in the class docstring

docs/source/examples/notebooks/simulations_and_experiments/custom-experiments.ipynb

@@ -31,7 +31,7 @@
    "source": [
     "## Custom termination\n",
     "\n",
-    "Termination of a step can be specified using a few standard strings (e.g. \"4.2V\" for voltage, \"1 A\" for current, \"C/2\" for C-rate), or via a custom termination step. The custom termination step can be specified based on any variable in the model.\n",
+    "Termination of a step can be specified using a few standard strings (e.g. \"4.2V\" for voltage, \"1 A\" for current, \"C/2\" for C-rate), or as an inequality on any model variable, referenced by name with `pybamm.CoupledVariable`. The threshold can be a number, a `pybamm.Parameter` or a `pybamm.InputParameter`.\n",
     "Below, we show an example where we specify a custom termination step based on keeping the anode potential above 0V, which is a common limit used to avoid lithium plating,"
    ]
   },
@@ -63,16 +63,9 @@
     "parameter_values = pybamm.ParameterValues(\"Chen2020\")\n",
     "\n",
     "\n",
-    "# Create a custom termination event for the anode potential cut-off at 0.02V\n",
+    "# Terminate when the anode potential drops to 0.02V\n",
     "# We use 0.02V instead of 0V to give a safety factor\n",
-    "def anode_potential_cutoff(variables):\n",
-    "    return variables[\"Anode potential [V]\"] - 0.02\n",
-    "\n",
-    "\n",
-    "# The CustomTermination class takes a name and function\n",
-    "anode_potential_termination = pybamm.step.CustomTermination(\n",
-    "    name=\"Anode potential cut-off [V]\", event_function=anode_potential_cutoff\n",
-    ")\n",
+    "anode_potential_termination = pybamm.CoupledVariable(\"Anode potential [V]\") < 0.02\n",
     "\n",
     "# Provide a list of termination events, each step will stop whenever the first\n",
     "# termination event is reached\n",

packages/pybamm/src/pybamm/discretisations/discretisation.py

@@ -300,23 +300,7 @@ def _resolve_coupled_variables_in_model(self, model):
         """Resolve CoupledVariables in rhs, algebraic, initial_conditions, and boundary_conditions."""
 
         def resolve_symbol(symbol):
-            if isinstance(symbol, pybamm.CoupledVariable):
-                if symbol.name not in model.variables:
-                    raise pybamm.DiscretisationError(
-                        f"CoupledVariable '{symbol.name}' not found in model.variables."
-                    )
-                return resolve_symbol(model.variables[symbol.name])
-            elif hasattr(symbol, "children") and symbol.children:
-                new_children = []
-                changed = False
-                for child in symbol.children:
-                    new_child = resolve_symbol(child)
-                    new_children.append(new_child)
-                    if new_child is not child:
-                        changed = True
-                if changed:
-                    return symbol.create_copy(new_children=new_children)
-            return symbol
+            return pybamm.SymbolProcessor.resolve(symbol, model.variables)
 
         for var, expr in list(model.rhs.items()):
             resolved = resolve_symbol(expr)

packages/pybamm/src/pybamm/experiment/step/__init__.py

@@ -1,5 +1,5 @@
 from .steps import *
 from .base_step import BaseStep, BaseStepExplicit, BaseStepImplicit
-from .step_termination import BaseTermination, CurrentTermination, VoltageTermination, CustomTermination, CRateTermination, CrateTermination, _read_termination
+from .step_termination import BaseTermination, CurrentTermination, VoltageTermination, SymbolicTermination, CustomTermination, CRateTermination, CrateTermination, _read_termination
 
 __all__ = ['base_step', 'step_termination', 'steps']